The present invention relates to fusion bonding, particularly to vacuum fusion bonding, and more particularly to a process involving vacuum pull down that enables increasing the bonding temperature to produce enhanced bonding.
Typical bonding processes require essentially optically flat substrates in order to assure uniform sealing. This is particularly the case in large substrate applications (e.g.,  greater than 4-inch substrates). The primary problem in bonding substrates without voids is that most substrates are non-uniform in flatness. This results in non-contact areas when substrates are aligned for bonding. The non-contact areas cause several problems in that gas or air can be entrapped in the interstitial space where the substrates are not in contact. This will lead to a void and non-bonded area and also has the potential of forcing the substrates apart preventing any bonding and/or misaligning the substrates which might be registered on the other.
The present invention provides a bonding process which overcomes the above-mentioned problems. The process includes incorporating vacuum pull down to ensure uniform surface contact during the bonding process. The process provides for effectively bonding arbitrary sized and shaped substrates, such as glass, plastics, alloys, etc., which have a moderate melting point with a gradual softening point curve. The process is especially effective in bonding applications wherein one or both of the substrates to be bonded contain microchannels or grooves to be embedded between the substrates, and which can be used to apply vacuum between the substrates during the bonding process. The vacuum pull down, in addition to providing a differential pressure, enables both holding and alignment of the substrates.
It is an object of the present invention to provide a vacuum fusion bonding process.
A further object of the invention is to provide a bonding process which incorporates vacuum pull down.
A further object of the invention is to provide vacuum pull down for enhanced bonding of substrates of various sizes and shapes.
Another object of the invention is to provide a bonding process incorporating vacuum pull down whereby substrates can be bonded without voids.
Another object of the invention is to provide a process for bonding substrates, which includes vacuum pull down of the substrates, thereby eliminating the entrapment of gas or air within the bonded substrates.
Another object of the invention is to provide an enhanced vacuum fusion bonding process for substrates having a moderate melting point with a gradual softening point curve, such as glass, some plastics, and certain alloys. Basically, the invention is a process incorporating vacuum pull down for enhanced fusion bonding of substrates. The process is particularly applicable for large substrate bonding, such as the conventionally utilized substrates or wafers which may have diameters up to and greater than four (4) inches. The process eliminates non-contact areas between the substrates and thus eliminates the prior need for optically flat substrate surfaces in order to assure uniform sealing during the bonding process. The bonding process of this invention utilizes an active vacuum source to evacuate the interstices between the substrates, while at the same time provides a positive force to hold or align the substrates during bonding.
The vacuum pull down of the substrates allows one to increase the temperature of the bonding process to ensure that the softening point of the substrates has been reached and small void areas are filled and come in contact with the opposing substrate. This process is particularly effective for bonding applications wherein one or more of the substrates contain microchannels or grooves that can be used to apply vacuum between the substrates during the thermal bonding cycle.